The present invention relates to an epicyclic gear mechanism which provides multiple speeds, more particularly one which provides two forward speeds and one reverse speed.
Epicyclic gear arrangements have been known in the art to offer distinct advantages when used in connection with gas turbine, steam turbine, diesel and other types of engines in automotive, aerospace, marine, industrial and other applications. Epicyclic gearing in general provides a more compact, in-line arrangement with potentially significant savings in space and weight. Rapid speed changes can be achieved without taking gears out of mesh. Smaller size, greater stiffness, and slower operational speed of the components can result in reduced noise and vibratory response and increased efficiency. As horsepower in parallel shaft boxes increases the components therein tend to increase in size and the economic advantage of utilizing epicyclic gearboxes instead becomes increasingly manifest.
For marine applications, for example, utilization of a relatively compact ship propulsion system incorporating an epicyclic gear mechanism can prove particularly advantageous in terms of cost, envelope and efficiency. For "boost/cruise" propulsion systems, wherein two different engines are used, current practice is to operate them one engine at a time. This conventional operational approach, one which is exclusively disjunctive, is less efficient and economical due to greater power inputs, than one which is inclusively disjunctive, wherein both engines can be operated at the same time. In this context epicyclic gear mechanisms may prove especially valuable for recent and future ships employing an advanced electric drive. A large, step-up, combining gear can be used in order to enable a single gas turbine to drive both a propulsion and a ship service generator at their design speeds (approximately 4,000 rpm and 6,000 rpm, respectively); however, this combining gear imposes severe ship arrangement penalties, especially with regard to length. Alternatively, an epicyclic gear arrangement would obviate the need for a combining gear and result in an engine/generator stack-up length well within practical bulkhead spacings.
Moreover, epicyclic gear arrangements can greatly improve fuel economy for single-engine ship propulsion systems. Many current ship propulsion systems with gas turbine engine prime movers employ controllable-pitch (CP) propellers and locked-train, double-reduction (LTDR) gears. The CP propellers are necessary in order to provide reversing capability; the LTDR gears match the engine speed to the propeller speed. The LTDR gear ratio is determined at the design point (full power) and operates at that fixed ratio over the entire range of operation. If the reversing function were performed elsewhere, fixed-pitch (FP) propellers would be used instead of CP propellers. CP propellers are more costly to manufacture and operate, heavier, more complex and less efficient than FP propellers. The fixed ratio of the current LTDR gears forces the single engine (used at endurance ship speed and below) to operate at turbine speeds well below optimum in terms of fuel economy. An epicyclic gear mechanism which provides a second speed ratio capability of the transmission would significantly improve fuel economy.
Conventional epicyclic gear arrangements provide input-output coaxiality, so as to permit in-line arrangement of the driving equipment and driven equipment. To some extent mechanism design, particularly bearing design, can be simplified, due in part to an offsetting effect of radial forces on the input shaft and output shaft. Design simplification can translate into design compactness, and the gearbox can be more closely coupled with the driving equipment or driven equipment.
However, this in-line arrangement is limited in some respects. It is notably characterized by a lack of versatility. When the output is coaxial and in-line with the input one input is located at one end and one output is located at the opposite end. Either end may be exclusively adaptable to the input function or the output function due to the particular arrangement of gears, bearings, clutches and brakes. In this regard conventional in-line arrangements tend to skew or imbalance the locations of the clutches, brakes and bearings to one side of the epicyclic gear, since one side is input-oriented and the other side is output-oriented. Also, the inherent compactness of epicyclic gearing may be somewhat counteracted by requisite in-line axial lengths for in-line functioning. These aspects negatively impact the design potential of the mechanism in terms of simplicity and often-concomitant compactness, which in turn militates against internal drive applicability in various contexts as well as against more proximate coupling with driving or driven equipment in general. For some applications radial output rather than axial output better suits spatial requirements.
Consequently, conventional in-line arrangements are locationally restrictive in terms of input and output connections and cannot readily accomodate more than one input member or more than one output member. Moreover, axial length as well as asymmetry or imbalance of the mechanism about the axis to either side of the epicyclic gear tends to neutralize or inhibit desired effects and characteristics.